Sampling device and method for measuring fluid flow and solute mass transport

ABSTRACT

A device and a method for measuring fluid flow and solute mass transport in flow systems includes a casing ( 1 ) having inlet ( 3 ) and outlet ( 9 ) openings and a fluid passageway therebetween, the casing containing at least one fluid permeable insoluble adsorbent matrix ( 7 ) and at least one tracer material ( 5, 5′ ) located in the fluid passageway. The tracer material ( 5,5′ ) is a fluid permeable partially soluble material which at least prior to installation is not physically or chemically bonded to the adsorbent matrix ( 7 ) and is either mixed with the adsorbent matrix or is located in at least one section ( 4,4′ ) of the casing ( 1 ) separate from but in contact with at least one section ( 6,6′ ) of the casing holding the insoluble adsorbent matrix ( 7,7′ ). The device is intended to be installed in a medium having a fluid path therein.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a device and a method for measuringfluid flow and solute mass transport in flow systems.

BACKGROUND OF THE INVENTION

The invention provides for the measurement of solute fluxes and viscousflow in liquid and partially saturated porous media. The solutes may beorganic or inorganic molecules that are dissolved and/or attached todispersed colloids in the liquid. More specifically, the inventionrelates to the monitoring of solute fluxes in water resources,including, but not limited to, variably saturated soils, sediments andgroundwater aquifers, surface water, water for industrial purposes, tapwater, drinking water and aqueous waste streams.

Monitoring for solutes in such water resources is often needed to verifyconcentrations of harmful substances in relation to specificenvironmental standards or reference levels as proclaimed by theregulating authorities. So, the solutes that are of interest for thepresent invention represents a very diverse group of organic andinorganic compounds. Depending on the specific environment, they mayinclude for example petroleum or tar-based compounds, halogenatedsolvents, heavy metals, macronutrients, radionuclides, biocides andtheir metabolites, surfactants, hormones, pharmaceutical products andtheir metabolites etc.

The displacement of solute mass in variously saturated porous mediaoccurs by the combination of convection, J_(m), diffusion, J_(D), andhydrodynamic dispersion, J_(h) (Van Genuchten en Wierenga, 1986). Theconvective transport is the displacement of the solute along with theviscous flow of the liquid, and is described by the equation:J_(m)=qC  (1)where J_(m) is the convective transport flux (g cm⁻² s⁻¹), q is thevolumetric flux density of a viscous fluid (cm³ cm⁻² s⁻¹) and C is thesolute concentration (g cm-3).

Solute diffusion results from random Brownian motion of molecules insolutions and in variously saturated media. When the concentrationgradient and the volumetric fluid content, θ, are constant, diffusion ofa non-sorbing solute may be described by Fick's first law:$\begin{matrix}{J_{D} = {{- \theta}\quad D_{m}\frac{\partial C}{\partial x}}} & (2)\end{matrix}$where J_(D) is the diffusive transport flux (g cm⁻² s⁻¹), θ is thevolumetric liquid content of the porous medium (cm³ cm⁻³), D_(m) is thediffusion coefficient of the porous medium (cm² s⁻¹), and x is the spacecoordinate. Diffusion in a semi-infinite solution or porous medium doesnot move the centre of mass of a solute, as the Brownian forces move themolecules away from the centre of mass in all directions. However, in afinite medium with heterogeneous boundaries, diffusion moves the centreof mass away from a source zone and towards a sink zone along theconcentration gradient in the medium.

The diffusion coefficient D_(m) in porous media is always smaller thanthe diffusion coefficient of the molecule in a free bulk liquid, D_(o).This is due to the tortuous pathway of the connecting pores, and, amongother things, van der Waals interactions with the solid surface. The twodiffusion coefficients D_(m) and D_(o) are linearly related:D _(m) =kθ(L/L _(e))² D _(o)  (3)where k is an empirical constant, L and L_(e) are the straight distancebetween two points and the pathway laid out by the pore system,respectively. Hence, the quadratic term in equation 3 accounts for thepore tortuosity. The relationship between D_(m) and θ is highlynon-linear, because the tortuosity increases with decreasing watercontent.

Hydrodynamic dispersion results from pore scale heterogeneity of thepore water velocity magnitude and direction. It has been shown that thedispersion effect may be described mathematically similarly to diffusivetransport: $\begin{matrix}{J_{h} = {{- \theta}\quad D_{h}\frac{\partial C}{\partial x}}} & (4)\end{matrix}$

Where J_(h) is the dispersive transport flux (g cm⁻² s⁻¹), and D_(h) isthe mechanical dispersion coefficient (cm² s⁻¹). The mechanicaldispersion coefficient is increasing with increasing fluid velocityaccording to the empirical relationship:D_(h)=λν″  (5)

Where λ is the dispersivity (cm), v is the pore water velocity (cm s⁻¹),that is approximated as q/θ, and n is an empirical parameter, normallyequal to 1 (Van Genuchten en Wierenga, 1986). From this relationship itfollows that the dispersive flux, unlike the diffusive flux term,vanishes for v→0.

Combining the three terms contributing to the displacement, anexpression for the total solute flux, J_(s), is obtained:$\begin{matrix}{J_{s} = {{{- \theta}\quad\left( {D_{h} + D_{m}} \right)\frac{\partial C}{\partial x}} + {qC}}} & (6)\end{matrix}$

Substitution of this equation into the equation of continuity for asolute that does not undergo irreversible reactions: $\begin{matrix}{{\frac{\partial}{\partial t}\left( {{\theta\quad C} + {\rho\quad S}} \right)} = {- \frac{\partial J_{s}}{\partial x}}} & (7)\end{matrix}$

Yields the general transport equation: $\begin{matrix}{{\frac{\partial}{\partial t}\left( {{\theta\quad C} + {\rho\quad S}} \right)} = {{- \frac{\partial}{\partial x}}\left( {{{\theta\left( {D_{m} + D_{h\quad}} \right)}\frac{\partial C}{\partial x}} - {qC}} \right)}} & (8)\end{matrix}$

Where t is time (s), ρ is the bulk density of the porous medium (gcm⁻³), and S is the amount of solute adsorbed to the solute phase (gg⁻¹). The simplest form in which sorption to the solid phase can berepresented is by instantaneous linear equilibrium conditions:S=K_(d)C  (9)

Where K_(d) is the slope of the linear isotherm (cm³ g⁻¹). Whenconsidering steady liquid flow in a homogenous porous medium, implyingthat θ and q are constant in space and time, the transport equationreduces to: $\begin{matrix}{\frac{\partial C}{\partial t} = {{\frac{\left( {D_{h} + D_{m}} \right)}{R}\frac{\partial^{2}C}{\partial x^{2}}} - {\frac{v}{R}\frac{\partial C}{\partial x}}}} & (10)\end{matrix}$

Where R is the retardation factor:R=1+ ^(ρK) ^(d)/θ  (11)

Equation 8 may be solved numerically for transient conditions, that is,θ and q varies both in time and space, and dynamic boundary conditions.Numerical solution codes are available both in one and two dimensions.Equation 10 may be solved analytically for constant boundary conditions(e.g. Wierenga and Van Genuchten, 1986).

Gaseous transport in unsaturated porous media occurs due to diffusionprocesses only, unless pressure gradients are present. The diffusionprocess may again be described by Fick's first law: $\begin{matrix}{J_{D} = {{- \theta}\quad D_{g}\frac{\partial C_{g}}{\partial x}}} & (12)\end{matrix}$

Where D_(g) is the gaseous diffusion coefficient and C_(g) is thepartial gas concentration in the gas filled pores. Analogue to solutediffusion, the gas diffusion is highly non-linear with respect to thevolume contributing to the displacement, i.e. air-filled porosity.Hence, it follows that at a high liquid filled porosity, liquiddisplacement and solute diffusion dominate the transport process, whileat high air-filled porosity, gas-diffusion will dominate thedisplacement of a volatile compound in the porous medium.

DESCRIPTION OF THE RELATED ART

The most used existing methods for sampling of organic and inorganicsubstances and/or solutes in soils are soil coring and suctionlysimeters. Soil coring simply means that a soil sample is dug out fromthe ground and analysed. An example of a suction lysimeter is disclosedin U.S. Pat. No. 5,035,149, wherein a porous receptacle is buried in theearth, and an air conduit extends from the earth surface into thereceptacle. By drawing a vacuum on the air conduit, soil solution isdrawn in from the surrounding soil through the porous walls and iscollected in the receptacle. A separate conduit for transferring thesoil solution sample brings the sample to the surface when positive airpressure is applied to the receptacle through the air conduit.

Groundwater sampling systems are enclosed in for example U.S. Pat. No.4,745,801 and U.S. Pat. No. 4,759,227 wherein the sampling apparatus isconnected to a remote data collection device located above the ground.They do not have an adsorbing material in the sampling device to gatherdata on the adsorption rates or chemical make-up of the surroundingmedia.

EP-A-1 094.311 discloses a fluid sampling device especially for samplingwater in order to detect parasites therein. The device comprises acartridge containing a granular filter media, which can be compacted andthrough which the fluid to be sampled flows. The granular filter mediahas in a compacted state a pore size less than that of the parasites.The parasites captured can be removed upon expanding and backwashing thegranular filter media. This method is clearly not suitable for samplingof organic and/or inorganic solutes, as the method relies on amechanical filtering process.

The methods described above have in common that the momentaneous soluteconcentration is measured. Also, these measurements do provide a soluteconcentration only, and the flux of the compound has to be estimatedseparately by estimating or calculating the fluid flux.

WO 00/70339 discloses a water sampling method and apparatus forextracting biological and chemical analytes from water. Discrete samplesare successively withdrawn with a pumping system from a body of waterand an extraction device extracts analyte from the discrete samples andintegrates the extracted samples.

For regulatory purposes, there is often interest to measure the soluteand/or liquid flux over a longer time period. With momentaneoustechniques, a time series of repetitive measurements must be taken toaccomplish this, because in most natural flowing media soluteconcentrations vary strongly with time. Repetitive sampling is a costlyoperation, hence in practice, monitoring programs, for example ingroundwater systems, have to compromise on the number of samples thatare taken over a given period.

It would therefore be advantageous to measure integrated solute fluxesover longer time periods. In addition, it would also be advantageous ifthe measuring device does not require power- and time consumingoperations, such as vacuum systems, pumping operations or other powerconsuming functions.

More recently, passive sampling devices have been introduced. Passivesampling devices make use of mass transfer from the sampled media intothe sampling device, by the passive utilisation of solute concentrationand/or hydraulic gradients.

There may be distinguished between passive samplers where the masstransfer occurs through a semi-permeable membrane (SPM), impermeable tothe sampled liquid but permeable to the investigated solutes (U.S. Pat.No. 5,996,423, WO 92/04646, U.S. Pat. No. 5,904,743). Mass transferthrough the semi-permeable membrane is governed by diffusion:$\begin{matrix}{J_{D} = {{- D_{SPM}}\frac{\partial C}{\partial x}}} & (13)\end{matrix}$where J_(D) is the flux of the diffusing solute through the SPM (g/cm⁻²s⁻¹), D_(SPM) is the diffusion coefficient of the solute in the membrane(cm² s⁻¹) , C is the concentration of the solute in the fluid (g/cm⁻³),and x is the distance parallel to the direction of diffusion. Theinterior of the sampling device usually consists of a non-polar liquidwith high solubility for the solutes that are monitored, thus creating athermodynamic gradient that controls the diffusion of non-polarcompounds in the sampled medium towards the liquid in the SPM device.

Generally, the technical procedure of a SPM device involves installationof the device in the medium of interest, allowing uptake of the solutesof interest over a certain time period, removing the SPM device, andretrieving the accumulated amount of the solute. Thereafter, theconcentration of the solute in the surrounding medium is calculatedusing either equilibrium values or diffusion parameters from equation13.

Several issues limit the validation and practical applicability ofsemi-permeable membrane devices. First, the diffusion coefficient in thesemi-permeable membranes is dependent on the molecular properties of thesolute. This implies that the diffusion coefficient for each singlemonitored compound should be calibrated individually (Huckins, 1999),which complicates the quantification of the solute concentration.Further, actual diffusion rates may be affected by bio-fouling of themembrane and/or non-ideal diffusion at higher concentrations.

Second, the cumulative diffusion through the SPM is linear over alimited time period only, and this linear period varies individually fordifferent solute molecules (Huckins, 1999). After equilibrating the SPMfor a certain period, the uptake of some compounds may still follow theinitial linear kinetics, but other compounds will have past the linearstage, hence, quantification of the solute concentration in the fluidmedium becomes less accurate, due to the non-linear response.

Third, the concentration of the solute in the fluid medium directlyadjacent to the SPM interface, is dependent on the flow rate (Gustavson,2000): when the flow rate is high enough to compensate for the masstransfer through the SPM, the mass transfer is diffusion-controlled. Butif the flow rate outside the SPM is lower than the mass diffusivetransport through the SPM, the solute concentration in the surroundingfluid medium will decrease and the mass transfer through the membranewill be controlled by the liquid flux in the surrounding fluid medium.In this case equation 13 is no longer valid, and the liquid flow rateneeds to be known to derive the solute concentration in the fluid mediumfrom the accumulated mass in the SPM device (Gustavson, 2000).

Fourth, SPM devices are not permeable to mobile colloids, as they areonly permeable to solutes that are in the free dissolved phase. Organicand/or inorganic colloids can be the primary hosts for transport ofstrongly sorbing compounds in variously or permanently saturated mediaand surface water. The sorbing compounds being for example radioactiveisotopes, heavy metals and a polar contaminants. For risk assessmentpurposes, it is important to quantify the entire displaced compound,whether it is in the free dissolved phase or not.

In the second type of passive sampling devices used in liquid systems,the flowing liquid can freely pass the interface between the sampledmedium and the sampling device. Examples of such passive samplingdevices are disclosed in U.S. Pat. No. 5,355,736 and in WO 01/33173).The sampling device is filled with an insoluble matrix to which thesolutes of interest are adsorbed. The accumulated amount of the solutein the sorbing device is, according to WO 01/33173, proposed to berelated to the total mass flux into the sampler $\begin{matrix}{J_{s} = \frac{M_{s}}{t_{d}A_{u}}} & (14)\end{matrix}$in which J_(s) is the mass flux, M_(s) is the accumulated amount of thesolute adsorbed by the permeable sorbing device, t_(d) is the timeperiod during which the sorbing device is exposed to the flowing medium,and A_(u) is an area normal to the direction of the fluid flow that isused to define the fluid flux into the unit.

In WO 01/33173, at least one fluid-soluble resident tracer is introducedin known amounts in the sorbing unit and sorbed on the sorbent beforeinstallation of the device in the fluid system. The document describesthe case of the sorbing device being a circular column. As fluid passesthe permeable device, part of the tracer is transported along with thepassing fluid. The method comprises the following set of equations thatrelates the amount of tracer that is left on the sorbent after a certainperiod of time, to the amount of fluid that has passed the device:$\begin{matrix}{{M_{r} = {\frac{2}{\pi}\left\lbrack {{\arcsin\left( \sqrt{1 - \xi^{2}} \right)} - {\xi\sqrt{1 - \xi^{2}}}} \right\rbrack}}{With}} & (15) \\{\xi = \frac{t_{d}q}{2r\quad\theta\quad R_{d}}} & (16)\end{matrix}$where M_(r) is the fraction of the initial mass of tracer remaining, andr is the radius of the sorbing matrix. Three important shortcomingslimit the set of conditions for which equation 15 is valid. First,tracer displacement does not only occur due to convective transport, butalso due to diffusive transport out of the sampling unit. Second,equation 15 and 16 are based on local equilibrium assumptions (LEA).And, third, transverse diffusion into the sampling unit effects theamount of solute sorbed, M_(s) (equation 14). Concerning the firstissue, it is easy to see that for q→0, M_(r) in equation 15 remainsconstant, implying no mass loss of the tracer for zero flow conditions.Hence, equation 15 ignores the effect of the loss of tracer mass fromthe device due to diffusion. From equation 10 it follows that therelative role of diffusion increases with decreasing liquid velocity, v.Thus, at low liquid velocities, the measurement of tracer loss from thedevice will be biased with respect to the amount of water that passedthe device.

The absolute value of the diffusive flux is proportional to 1/R, butthis relation cannot be used to reduce the above-described bias, becausethe convective flux is also proportional to 1/R (equation 10). Hence,the relative error is the same regardless of the specific R-value of thetracer. Besides, tracers with very high R-values are not very useful forthe device described in WO 01/33173, because the relative mass lossoccurs very slowly, so that the difference of the initial and finalamount M_(r) cannot be measured accurately.

Second, calculation of the fluid flux (eq. 15-16) relies on the LEAassumption, meaning that instantaneous, linear sorption equilibrium ofthe tracer is reached at all times (see equation 9). However, LEA isoften not valid due to either i) nonlinearity of the sorption isotherm;ii) rate limitations due to surface reactions and/or diffusion indead-end pores; iii) competition effects, which is important for examplefor ion-exchange processes; and iv) adsorbent heterogeneity. With qincreasing, there comes inevitably a point where the LEA no longerholds, this point representing the higher limit for the application ofequation 15, 16. Under normal flow rates, due to the considerationsabove, the LEA is valid only in macroporous adsorbents. Small, dead-endmeso-pores (<50 nm) or micropores (<2 nm) give rise to rate-limiteddiffusion and high-energy adsorption interactions. These processesinvalidate the LEA. But, on the other hand these pores are of highimportance because they give rise to high surface areas that stronglyincrease sorption affinity for solutes of interest, and protect sorbedorganic compounds from being biodegraded by micro-organisms migratingalong with the fluid. Adsorbent heterogeneity arises when severaladsorbents are mixed in the sampling device. In this case, not onesingle K_(d) value may be used and the LEA is no longer valid.

A third problem, is that also the transport of solutes in the pore-waterinto the sorbing device will be biased by transverse diffusion at lowerpore water velocities. In fact, some devices make use of this transversediffusion to derive approximations for the solute concentration in thepore water (e.g. Harper et al. 1997). Diffusion and hydrodynamicdispersion occur transverse to the flow direction through the interfaceof the sorbing device that is parallel with the flow direction, becausethere is a constant concentration gradient into the sampling device.While the concentration in the surrounding fluid is C, the soluteconcentration in the fluid inside the sorbing device will be close tozero, due to the strong sorption properties for the solute of interest.

This transverse mass transfer into the sorbing device is a source oferror that invalidates equation 14 in the case that the contribution issignificant in comparison to the convective transport of the solute intothe sorbing device. The diffusive/dispersive contribution to solutedisplacement is affected by the diffusion coefficient and concentrationgradient in the surrounding medium, while the convective part iscontrolled by v (equation 6, 8, 10). When taking the discretized form ofequation 6, and assuming that the liquid concentration of the poresolute concentration inside the sorbing device is close to zero due tostrong sorptive interactions(ΔC≈C), and conservatively assuming thathydrodynamical dispersion is negligible, it follows that the diffusiveflux across the interface of the surrounding medium and the sorbingdevice, and the convective flux are of equal magnitude if the followingequation applies: $\begin{matrix}{\frac{\left( D_{m} \right)}{\Delta\quad x} = v} & (17)\end{matrix}$

Here, Δx is the finite diffusion distance over the interface between thesurrounding medium and the adsorbent in the sampler The Δx is equal tothe thickness of the permeable screen surrounding the sorbing device. Asan example, molecular diffusion coefficients of charged ions in aqueoussolutions are typically around 1e-5 cm²s⁻¹ (Kemper, 1986). Assuming thatΔx=0.2 cm, this would mean that equation 17 applies if the linear fluidvelocity is 5e-5 cm s⁻¹, equal to 43 mm day⁻¹.

In a porous medium, the transverse diffusion from the fluid towards thesorbing devices will then be controlled by the effective bulk diffusionand the second derivative of the concentration with respect to space(equation 10). So, it follows that the error will be largest for soluteswith low sorption affinity with the surrounding porous medium. In aliquid, the diffusion towards the sampler will be governed by D_(o)only.

An example of a porous medium were adverse diffusion may invalidateequation 14 and 15 may be taken in the recharge of precipitation to thegroundwater in variably saturated soils. Annual groundwater rechargewill typically be below 600 mm year⁻¹. So, the average flux below theroot zone towards the groundwater will be less than 1.6 mm day⁻¹. Withthe volumetric water content, θ, usually in the range of 0.1-0.3, itfollows that average pore water velocities in variably saturated soilstypically are below 16 mm day⁻¹, which is below the critical valuederived above. Hence, in the case of groundwater recharge the resultsmay be strongly influenced by diffusion processes.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a device allowingaccurate measurements of fluid flow and solute mass transport in flowsystems. The device according to the invention comprises a casing havinginlet and outlet openings and a fluid passageway therebetween, saidcasing containing at least one fluid permeable insoluble adsorbentmatrix and at least one tracer material, said tracer material is a fluidpermeable partially soluble material which at least prior toinstallation is not physically or chemically bonded to the adsorbentmatrix.

According to one embodiment of the invention the tracer material islocated in at least one section of the casing separate from but incontact with at least one section of the casing holding said insolubleadsorbent matrix and that the at least one tracer material section andthe at least one adsorbent matrix section are located in the fluidpassageway between said inlet and outlet openings. In such an embodimentthe tracer material may or may not be physically or chemically bonded tothe adsorbent matrix after fluid contact when it starts to dissolve inthe fluid and move into the adsorbent matrix section.

In one embodiment of the invention the device comprises at least twotracer material sections spaced apart and an adsorbent matrix sectiontherebetween, which is in capillary contact with said at least twotracer material sections. With such a device the diffusion contributionto the tracer mass displacement can be compensated for.

This may also be done with an alternative embodiment of the invention,in which the device comprises at least two adsorbent matrix sectionsspaced apart and with a tracer material section therebetween, which isin capillary contact with said at least two adsorbent matrix sections.

According to an alternative embodiment the tracer material and theadsorbent matrix are located in the same section in the casing, whereinthe tracer material and the adsorbent matrix are macroscopically mixedwith each other. In such an embodiment the tracer material should not bephysically or chemically bonded to the adsorbent matrix neither beforenor after fluid contact when it starts to dissolve in the fluid.

The tracer material may be chosen from the following groups ofmaterials: inorganic, organic and hybrid organic/inorganic salts;organic, inorganic or hybrid organic/inorganic solids, includingpolymers, copolymers, block copolymers and oligomers in which hydrolysisof certain bonds can lead to the loss of part of the material;microencapsulated materials in which the tracer with controlled rate isreleased from the encapsulation in fluids to be measured. According toone aspect of the invention the tracer material is a salt having asolubility product (K_(sp)) in the fluid in question of between 10⁻² and10⁻⁶⁰ , preferably between 10⁻² and 10⁻⁴⁰ and more preferably between10⁻⁵ and 10⁻¹².

The adsorbent matrix material is an organic, inorganic or hybridorganic/inorganic material having adsorbent properties. According to oneaspect of the invention the adsorbent matrix material is chosen from thefollowing groups of materials: silica, aluminium silicate, aluminiumzirconium, metal oxides, synthetic ion exchange resins, carbonaceousmaterials, zeolites, cellulose, synthetic polymeric materials. In oneembodiment of the invention the adsorbent matrix material is a hexagonalmesoporous silica.

According to one embodiment the casing comprises an inlet section and anoutlet section adjacent said inlet and outlet openings and respectively,said inlet and outlet sections being located outside and presenting aninterfacial area to said adsorbent matrix or tracer material sectionsand wherein the projected area normal to the flow direction thatcontributes to momentum flow into the permeable unit through said inletopening differs in size from said interfacial area between the inletsection and the adsorbent section or the tracer section.

The device is preferably arranged in a housing, which is impermeable orpermeable to fluid. The casing may be removed from the housing forfurther analysis in the laboratory.

According to one embodiment two or more devices each comprising a casingwith at least one adsorbent matrix section and at least one tracermaterial sections and inlet and outlet openings are arranged in the samehousing. The casings may or may not be angularly displaced with respectto each other. In the latter case they may contain different tracermaterials and/or adsorbent matrix materials.

According to another embodiment the housing is rotatably connected to acore portion having magnetic properties, said core portion beingrotatably connected to a cable or rod intended for the installation ofthe device.

According to still another embodiment the housing is provided with amember having a hydrodynamic shape, said housing being rotatablyconnected to a cable or rod intended for the installation of the device.

In another aspect of the invention the housing is part of ahigh-frequency waveguide configuration that connects to a coaxial cable.

The invention also provides a method of measuring fluid flow and solutemass transport in flow systems, comprising:

installing in a medium having a fluid path therein a device comprising acasing having inlet and outlet openings and a fluid passagewaytherebetween, said casing containing at least one fluid permeableinsoluble adsorbent matrix and at least one tracer material, allowingfluid to pass from said inlet opening through the adsorbent matrix andtracer material to said outlet opening, wherein the tracer material is afluid permeable partially soluble material which initially is notphysically or chemically bonded to the adsorbent matrix and that uponfluid contact the tracer material will dissolve in the fluid and thatafter a sampling period the device is removed from the flow system andthe amount of tracer material residing in the adsorbent matrix isquantitatively measured to derive therefrom the fluid flow through thedevice.

According to one embodiment the tracer material is located in at leastone section of the casing separate from but in contact with at least onesection of the casing holding said insoluble adsorbent matrix, so thatupon fluid contact the tracer material will dissolve in the fluid and bedisplaced into the adsorbent matrix section, and that after a samplingperiod the device is removed from the flow system and the amount oftracer material displaced into the adsorbent matrix is quantitativelymeasured to derive therefrom the fluid flow through the device.

According to one aspect of the invention solute adsorbed to theadsorbent matrix is quantitatively measured to derive therefrom theconcentration of solutes in the fluid flow.

According to a further aspect of the invention the amount of tracermaterial displaced into the adsorbent matrix in a direction opposite tothe flow direction is quantitatively measured to compensate fordiffusion contribution to the tracer mass displacement.

According to still another aspect the amount of tracer materialremaining in the tracer section is quantitatively measured and comparedwith the amount of tracer material displaced into the adsorbent matrixto derive therefrom diffusion contribution.

According to an alternative embodiment the tracer material and theadsorbent matrix are located in the same section in the casing, whereinthe tracer material and the adsorbent matrix are mixed with each other,and that upon fluid contact the tracer material will dissolve in thefluid and that after a sampling period the device is removed from theflow system and the amount of tracer material left in the casing isquantitatively measured to derive therefrom the fluid flow through thedevice.

In one aspect of the invention the tracer materials a dispersedinorganic, organic or hybrid inorganic/organic salt of which thepositive cation and negative anion do not have sorption affinity forsaid at least one insoluble adsorbent matrix, such that upon fluidcontact the tracer salt will dissolve into the fluid according to itssolubility product, and that after a sampling period the device isremoved from the flow system and the amount of tracer material left inthe casing is quantitatively measured to derive therefrom the fluid flowthrough the device.

In a further aspect of the invention the method comprises the furtherstep of quantitatively measuring recovery standard material adsorbed tothe adsorbent matrix simultaneously with quantitatively measuring soluteadsorbed to the matrix, to derive therefrom a more precise measurementof the amount of solute adsorbed to the adsorbent.

According to one embodiment the method comprises: installing in saidmedium at different locations thereof, especially different depths, twoor more devices each comprising a casing having inlet and outletopenings and at least one adsorbent matrix and at least one tracermaterial, and that after a sampling period the devices are removed fromthe medium and analysed separately. Said different devices installed maycontain different tracer materials and/or adsorbent matrix materials, toenable detection of different types of solutes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be further described in non-limitingway under reference to the accompanying drawings in which:

FIG. 1 schematically illustrates one embodiment of a sampling deviceaccording to the invention.

FIG. 2 illustrates another embodiment of a sampling device according tothe invention.

FIG. 3 illustrates a further embodiment of a sampling device accordingto the invention.

FIG. 4 illustrates an embodiment of a sampling device adapted for lowflow systems.

FIG. 5 illustrates an embodiment of a sampling device placed at an angleto the flow direction adapted to low flow systems.

FIG. 6 illustrates another embodiment of the sampling device adapted forhigh flow systems.

FIG. 7 illustrates an embodiment of a sampling device according to theinvention adapted for flow systems in which the flow direction isconstant in time.

FIG. 8 illustrates an embodiment of a sampling device according to theinvention adapted for flow systems in which the flow direction is notknown.

FIG. 9 illustrates an embodiment of a sampling device according to theinvention adapted for high flow systems with shifting flow directions.

FIG. 10 illustrates an embodiment wherein a sampling device is installedin a bypass conduit.

FIG. 11 illustrates an embodiment wherein several sampling devices areinstalled at different depths.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The device shown in FIG. 1 represents a simple technical configurationof the sampling device according to the invention. The device contains acasing 1 comprising an inlet section 2 having an inlet opening 3, whichpreferably is covered with a mesh, a perforated screen or the like, afirst permeable tracer section 4 containing at least one partly solubleinternal tracer 5 in a known amount and with known diffusion properties,an adsorbent matrix section 6 in the form of a permeable volume filledwith an insoluble solid porous matrix 7, comprising of at least oneadsorbent material that is particularly suited for the solutes ofinterest and the used tracers, a second permeable tracer section 4′containing at least one partly soluble internal tracer 5′ in a knownamount and with known diffusion properties, an outlet section 8 havingan outlet opening 9 preferably covered with a mesh, a perforated plateor the like, and a solid housing 10 for the casing 1. The housing 10 mayeither be made of any appropriate impermeable material such as stainlesssteel or a polymer material, such as PTFE or PVC, but the housing 10 mayalso be made of permeable metallic compounds or organic materials. Thelatter may be preferred in order to minimize the disturbance of the flowpaths of the liquid into the inlet section 2. The casing 1 enclosing thetracer section(s) and adsorbent matrix section(s) is permeable to theliquid only through the inlet and outlet openings 3 and 9. The casingholding the tracer section(s) and adsorbent matrix section(s) is alwaysphysically separated from the housing for further analysis in alaboratory.

The areas depicted m₁ and m₂ schematically illustrate the massdistribution of the tracer displaced into the adsorbent matrix.

The inlet section 2 may contain a non-reactive solid phase matrix suchas for example woven mesh material, glass beads, or silica flour ofdifferent particle dimensions, which has the function:

-   -   in porous media to attain capillary contact of the inlet section        with the surrounding porous medium while maintaining        sufficiently high fluid conductivity    -   in high-flow environments to transform turbulent flow of the        entering liquid into laminar flow of the liquid towards the        permeable tracer and adsorbent matrix sections 4, 4′ and 6.    -   Capture mobile colloids and avoid clogging of the permeable        tracer and adsorbent sections.

As previously mentioned, quantitative monitoring of the cumulativepassage of fluid, for example groundwater, is of prime importance in thedevice. This quantification can be achieved by using a simple andprecise mass balance system based on the dissolution/mobilization ofsuitable tracers that are initially present in at least one tracersection. The tracer material is preferably, but not limited to, anorganic or inorganic or a hybrid organic/inorganic salt that has asolubility product between 10⁻² and 10⁻⁴⁰, with a preferred rangebetween 10⁻⁵ and 10⁻¹². The solubility product controls the equilibriumof the solid phase with the solution phase, so that always a constantconcentration in the solution phase is maintained as long as the solidphase is in contact with solution. Therefore, salts as a tracer sourcediffer fundamentally from adsorbed tracers. From inspection of equation9, it is clear that for sorption reactions, the solution concentrationis dependent on the sorbed concentration and vice versa. Tracersubstances should always meet the following criteria:

-   -   The tracer is harmless to the fluid environment in which the        sampler is installed    -   The tracer goes into solution to a constant concentration of the        tracer in the passing fluid, regardless of the fluid velocity.    -   Tracer material is present in negligible amounts in the        surrounding fluid environment to which the device is exposed.    -   The tracer may be effectively retrieved from the adsorbent        matrix after the installation and measured with standard        analytical techniques.    -   The following classes of tracers may be distinguished:

A. Tracers based on inorganic salts having a low solubility in water.The advantages of inorganic salts are numerous:

-   -   The solubility products (K_(sp)) for salt ←→ ion equilibria are        well known and well defined over a range of temperatures, and        change little of the range of temperatures in the environment of        the sampler.    -   There exist many inorganic salts for which K_(sp) values are        sufficiently low so that 100% solubilizing of the salt out of        the sampler would not occur, but just enough so that the        quantity of water passing through the sampler could be measured.    -   It is easy to employ a combination of inorganic salts so that        the error in the quantification of one ion could be compensated        by measuring the amount of the other.    -   Extraction of the remaining ions can be done easily in under        laboratory conditions and their concentration can be simply        determined by standard analysis methods.    -   These salts are not affected by microbial degradation.    -   These salts are very cheap and are produced in high purity in a        large scale.    -   They can be easily incorporated by mutual grinding/mixing into        the device    -   There are plenty of examples salts that are harmless towards the        environment and have low K_(sp) values, for example: BaF₂        (K_(sp) 1.84×10⁻⁷); BaSO₄ (1.08×10⁻¹⁰); BaCO₃ (2.58×10⁻⁹); CaCO₃        (4.96×10⁻⁹); CaF₂ (1.46×10⁻¹⁰); CaSO₄ (7.10×10⁻⁵); Ca(OH)₂        (4.68×10⁻⁶); KClO₄ (1.05×10⁻²); MgCO₃ (6.82×10⁻⁶); MgF₂        (7.42×10⁻¹¹); MgNH₄PO₄ (2.0×10⁻¹³); Mg(OH)₂ (5.61×10⁻¹¹); MnS        (4.65×10⁻¹⁴); Zn(OH)₂ (4.13×10⁻¹⁷); ZnS(K_(sp)2.93×10⁻²⁵);

B. Tracers based on hybrid organic/inorganic salts. Here, the tracercontains an organic anion and an inorganic cation, or vice versa. Theorganic ion can, after the dissolution in water, be adsorbed into theadsorbent section of the device and then extracted with the otherorganic solutes. Likewise, the inorganic ion can be adsorbed, extractedand quantitatively analyzed. Generic structures for hybrid salts couldinclude a suitable cation, for example Ba²⁺, and an organic ion such asstearate, oleate, or amine. Both the cation and the anion could bemonitored in this case. Of course, one could also choose to monitor onlythe organic or the inorganic tracer. Also, almost any carboxylic acidthat it is not too soluble in water can be used. Salts from carboxylicacids are very simple to produce, and there is a large potential ofapplication, since the range of carboxylic acids is huge.

C. Tracers based on organic/organic salts. Here, an organic cation (forexample an ammonium salts such as tetrabutyl ammonium) would be coupledto an organic anion (such as acetate, pivalate, stearate etc). Again, itwould be possible to retain either one or both of the ions in theadsorbent section. Also this type of salts should be selected withrespect to favorable dissolution properties, crystal size, sensitivityto pH, and production costs.

D. Tracers based on organic polymers. Here, the tracer displacement isbased on the breaking of co-valent bonds by hydrolysis. Possible bondsinclude, but are not limited to, esters and amides. Polymers can be usedthat are slightly soluble in water, such as certain (poly)lactic acidsor certain (poly)amino acids. Also, block copolymers can be used from apolymer that is soluble in water and one that is not soluble, forexample (poly)lactic acid and (poly)ethylene.

E. Microencapsulated materials in which the tracer with controlled rateis released from the encapsulation in aqueous fluids.

To give some examples, we observed that the dissolution rates of CaF₂,Ca-Citrate, CaHPO₄, Ca-oleate and Ca-laurate into water were fast enoughto reach constant equilibrium values in a variably flow regime in therange from 5 to 128 mm/hr. So, the amount of these salts displacedrepresented a direct measure for the amount of water passing thesampler, regardless of fluid velocity. This holds as long as diffusionprocesses may be neglected a device as in FIG. 3 may be used. At lowerflow rates, when the role of diffusion becomes more important, a deviceas in FIG. 1 may be used.

When choosing a salt for the embodiment in FIG. 3, wherein the tracer ismixed with the adsorbent matrix, a salt that does not or that onlyinsignificantly will sorb to the adsorbent matrix before fluid contactas well as after fluid contact, when it has been dissolved by the fluid,should be chosen. One example of a salt from which the ions of thedissolved salt will not sorb or only insignificantly will sorb tocommonly used adsorbent matrixes is CaF₂. For the embodiment in forexample FIGS. 1 and 2, wherein the tracer material section(s) is/areseparate from the adsorbent matrix section, a salt may be chosen whichafter fluid contact either will not or will sorb to the adsorbentmatrix. For example, sorption interaction include non-specific bondinglike Loondon-van der Waals interactions, hydrogen bonding, hydrophobicinteraction, but also includes specific bonding such as ligand exchange,coordinated bonding or ion exchange. Examples of salts that once insolution will adsorb to commonly used adsorbent matrixes are Ca-oleateand Ca-laurate, and also phosphates, like CaHPO₄. In these examples itis the anion that will adsorb.

In the device according to FIG. 1, the first tracer 5 moves downstreaminto the adsorbent matrix section 6 unit as the sum of momentum anddiffusion transport, while the second tracer 5′ is displaced upstreaminto the adsorbent matrix section 6 as a result of diffusioncounteracting liquid transport. The tracer and adsorbent properties arechosen in such a way, that all the tracer mass flowing into theadsorbent matrix section 6 is retained by the adsorbent matrix 7 withinthe desirable installation time in the fluid system. The first andsecond tracers 5 and 5′ are preferably chemically different. The use oftwo different tracers is beneficial for the configuration as in FIG. 1,if one tracer is present in the upstream section of the adsorbentsection, and the other in the downstream tracer section.

It is also possible to quantitatively measure the amount of tracermaterial 5 remaining in the tracer section 4 and compare with the amountof tracer material displaced into the adsorbent matrix 6 to derivetherefrom diffusion contribution. In this case it would be sufficient touse a modified sampling device according to FIG. 1 with only one tracersection 4.

In the device shown in FIG. 2 one permeable tracer section 4 is placedin the middle of the adsorbent matrix section thus dividing it intofirst and second adsorbent matrix sections 6 and 6′. This can bebeneficial for avoiding release of tracer into the surrounding flowsystem. Upon fluid contact tracer 5 is displaced downstream into thesecond adsorbent matrix section 6′ as the sum of momentum diffusiontransport and is displaced and upstream into the first absorbent section6 as a result of diffusion counteracting liquid transport. The tracerdisplaced into the respective first and second adsorbent matrix sections6 and 6′ is analysed.

The fluid conductivity of the permeable device is controlled such thatthe fluid enters the tracer section with a linear velocity v (cm s⁻¹).Consider the displacement of a tracer downstream into the adsorbentmatrix section 6, with the concentration of the tracer C at theinterface between the tracer section and the adsorbent matrix section isconstant, C_(o), and the initial amount of the tracer in the adsorbentmatrix section 3 is zero. Then, initial and boundary conditions are:C(x,0)=0  (18)C(0,t)=C _(o) H(t),C(∞,t)=0where C_(o) is the concentration of the tracer at the interface of thetracer section and the adsorbent matrix section, and H is the heavysidestep function. The adsorbent matrix section is assumed to be ofsemi-infinite length. The solution is well known: $\begin{matrix}{\frac{C\left( {x,t} \right)}{C_{o}} = {{\frac{1}{2}{{erfc}\left\lbrack \frac{{Rx} - {vt}}{2\sqrt{DRt}} \right\rbrack}} + {\frac{1}{2}{\exp\left\lbrack \frac{vx}{D} \right\rbrack}{{erfc}\left\lbrack \frac{{Rx} - {vt}}{2\sqrt{DRt}} \right\rbrack}}}} & (19)\end{matrix}$

Where erfc is the complementary error function, and x is the distanceincreasing in the flow direction. For diffusion of the solute upstreamin the adsorbing section, the solution is the same equation 19, exceptthat v is negative:ν_(upstream)=−ν_(dowstream)  (20)

If the entire tracer mass that is displaced into the adsorbent sectionremains, due to sorption, in the adsorbent section during theinstallation period of the device, the sampler can be regarded as asemi-infinite medium, and the net result of diffusion to thedisplacement of the centre of mass is zero. Hence, a very usefulproperty may be derived by numerically taking the area under the curvesin FIGS. 1 and 2, m₁ and m₂, representing the mass displaced in thedownstream and upstream direction, respectively. The mass m₁ and m₂ isschematically depicted in FIGS. 1 and 2. It follows that:$\begin{matrix}{v = {{\frac{R}{\left( {C_{o}T} \right)}\left\lbrack {{\sum\limits_{x = 0}^{l}\quad{C_{x}\Delta\quad x}} - {\sum\limits_{x = {- 1}}^{0}\quad{C_{x}\Delta\quad x}}} \right\rbrack} \equiv {\frac{R}{\left( {C_{o}T} \right)}\left( {m_{1} - m_{2}} \right)}}} & (21)\end{matrix}$

Where l is the distance from the source of the tracer to the end of theadsorbent section, and m₁ and m₂ have the unit g cm⁻². Using theproperty: $\begin{matrix}{v = \frac{V}{AT}} & (22)\end{matrix}$

It follows that: $\begin{matrix}{V = {\frac{R}{C_{o}}\left( {M_{1} - M_{2}} \right)}} & (23)\end{matrix}$

Where M₁ and M₂ is the mass displaced in downstream and upstreamdirection, respectively, now having the unit g. Thus, V may be derivedfrom the mass of a tracer that is displaced upstream and downstreamduring the installation period, irrespective of the porewater velocityv. It follows from equation 23 that V=0 when M₁=M₂ (fully diffusioncontrolled) and for M₂→0, the solute displacement is fully controlled byconvective (momentum) transport.

Likewise, it may be shown that V is a linear function of the mass oftracer a displaced in downstream direction and tracer b in upstreamreaction given by: $\begin{matrix}{{{qV} + r} = {{\frac{R_{a}}{C_{o,a}}M_{1,a}} - {\frac{R_{b}}{C_{0,b}}M_{2,b}}}} & (24)\end{matrix}$where q and r are calibration parameters. The use of two differenttracers is beneficial for the configuration as in FIG. 1, if one traceris present in the upstream section of the adsorbent section, and theother in the downstream tracer section.

Equation 23 and 24 are used to derive the liquid flow volume in casethat diffusive loss from the tracer section cannot be ignored. But inaddition, due to the separated tracer source and adsorbent sections, itis possible to obtain another independent mass balance to verify whethercounterflow diffusive loss out of the tracer section occurred during theinstallation period. This is simply done by extracting, after theinstallation period, both the remaining tracer in the upstream tracersection, and the accumulated tracer amount in the adsorbent section. Ifthe sum of these two equal the initial amount of the tracer beforeinstallation, it is obvious that counterflow diffusion may be ignored.In this case, the volume of the liquid may be simply derived from thesolubility product of the tracer substance.

FIG. 3 shows an alternative embodiment in which the the tracer material5 and the adsorbent matrix material 7 are located mixed with each otherin the same section in the casing 1. The tracer material is notphysically or chemically bonded to the adsorbent matrix neither beforenor after fluid contact when it starts to dissolve in the fluid. Thetracer material should be chosen in accordance herewith.

Preferably the tracer material 5 in this case is a salt. According toone alternative it is a dispersed inorganic, organic or hybridinorganic/organic salt of which the positive cation and negative aniondo not have sorption affinity for the adsorbent matrix, such that uponfluid contact the tracer salt will dissolve into the fluid according toits solubility product. After a sampling period the device is removedfrom the flow system and the amount of tracer material left in thecasing is quantitatively measured to derive therefrom the fluid flowthrough the device.

Quantification of the solute(s) adsorbed by the adsorbent matrix is donein laboratory with routinely available methods, always involving anextraction step and a detection step. The extraction step can be donefor example by batch shaking extraction, soxhlet extraction or usingvacuum manifold extraction stations. Subsequent detection of the solutesis for example done by GC-MS (Gas Chromatography-Mass Spectrometry),HPLC-MS (High Pressure Liquid Chromatography-Mass Spectrometry) or ICP(Inductively Coupled Plasma Emission Spectrometry). An internal recoverystandard improves the precision of these procedures, and is requiredwhen performing MS quantification. When initially sorbed to theadsorbent matrix, an appropriate internal standard will control forextraction and detection variability. One or more internal standardsshould be chosen depending on the specific detection method and therange of molecules to be detected. The best internal standard for MSdetection is an isotopically labelled version of the solute to bequantified, because it will have a similar extraction recovery,ionisation response, and a similar chromatographic retention time.

The analytical accuracy of the measuring device may be further improvedby regulation of the velocity v with which the liquid enters upstreaminto the tracer section. In low flow systems, it may be desirable toconverge the liquid flow paths in the inlet such that the linear flowvelocity in the tracer and adsorbent units are increased. This has threemajor objectives:

-   -   To reduce the diffusion contribution to the tracer mass        displacement from the tracer source section(s).    -   To increase effective sampling volume of the surrounding medium        contributing to fluid transport into the unit, which is        desirable if the fluid flow in the surrounding medium is        heterogeneous.    -   To increase the absolute volume V and mass M that pass the        permeable unit within a certain installation period, which is        desirable when a low detection limit for the solute under        investigation is needed.

To achieve these objectives a device as shown in FIG. 4 or 5 may beused, said device is designed such that the projected area A>B, where Ais the projected area normal to the flow direction that contributes tomomentum flow into the inlet section 2, and B is the interfacial areabetween the inlet section 2 and the tracer section 4. Hence, the flowpath of the liquid converge and the linear velocity v at theintersection B is A/B times higher than the velocity v of the liquid atthe interface of the surrounding medium and the inlet of the samplingdevice. This assumes that the hydraulic conductivity of the permeablesection does not limit the flow into the permeable unit.

In case the sorbing unit is installed perpendicular to the flowdirection A=A′, where A′ is the area in direct contact with the flowingliquid/porous medium (FIG. 3). Alternatively the sorbing unit may beinstalled in an angle to the flow direction, such that A=A′cos(α), whereα is the angle of installation, as is shown in FIG. 5.

In high flow systems, it may be desirable to diverge the liquid flowpaths in the inlet such that the linear flow velocity in the tracer andadsorbent units are decreased. This has three major objectives:

-   -   To avoid turbulent flow and high hydrodynamic dispersion in the        tracer and the adsorbent sections of the device.    -   To assure that the tracer dissolution may reach equilibrium with        the passing fluid    -   To increase the residence time of the liquid in the adsorbent        unit, such that all of the passing solutes can be adsorbed.

To achieve these objections, the unit is designed such that theprojected area A<B, as is shown in FIG. 5) where A is again theprojected area normal to the flow direction that contributes to momentumflow into the permeable unit, and B is the interfacial area between theinlet section 2 and the tracer section 4. Hence, the flow path of theliquid diverge and the linear velocity v at the intersection B is A/Btimes smaller than the velocity of the liquid at the interface of thesurrounding medium and the inlet of the sampling device. The sorbingunit may be either installed normal to the flow direction; in this caseA=A′, where A′ is the area in direct contact with the flowingliquid/porous medium. Or, the sorbing unit may be installed in an angleto the flow direction, such that A=A′cos(α), where α is the angle ofinstallation.

In flow systems where the liquid flow direction is known a priori, thepermeable unit is oriented parallel or in an angle to the flowdirection, however always such that an inlet and an outlet are exposedto the surrounding fluid or porous medium in the upstream and downstreamdirection, respectively. It is in most cases important that the positionof the permeable unit is fixed during the installation. This orientationand fixation may be attained by different ways, depending on the flowenvironment.

Typical low flow environments include variously saturated soils andpacked bed reactors, for which the flow direction is usually known. Adevice such as in FIG. 5 may then be installed by making a bore hole 11with standard auger equipment, and inserting the device into the borehole 11. The inlet and outlet section 2 and 8 should obviously be incapillary contact with the surrounding soil, and positioned in theupstream and downstream direction of the fluid flow, respectively. Theangle of installation to the main flow direction is then recorded.

In high flow pipelines or tubings, the fluid flow direction is alsoknown in advance, and the device may be attached to the interior of thepipeline or be installed as an inline configuration. Alternatively, theinstallation may be done by constructing a bypass pipeline or tubing 12,which contains a smaller part of the total liquid flow through apipeline 13, for example a drinking water pipeline, as shown in FIG. 10,or in a large container, for example a freshwater container.

In other systems, like groundwater aquifers, medium-velocity horizontaltransport (≈10-200 m year) typically occurs, and the flow direction isnormally constant in time, and may be measured using other devicesbefore installing the device. An example of a suitable device is shownin FIG. 7 and comprises a core portion 14 having magnetic properties,said core portion 14 is rotatably connected to a housing 10 holding atleast one sampling device 1 with inlet and outlet openings 3 and 9. Inthis case, the device may be installed in a vertical borehole casing,and may be fixated by giving the core portion 14 magnetic properties.The inlet and the outlet openings 3 and 9 are in a desired fixedposition with respect to spatial coordinates during the installation, asthe housing 10 holding the sampling device 1 can be rotated with respectto the core portion 14. The core portion 14 can rotate freely into thefixed coordinate position due to a rotation joint 15 connected to acable or rod 16 that is used to place the device at a certain depth inthe liquid system.

If the fluid flow direction in medium or low-flow environments is notknown a priori, the device may consist of two or more, for examplethree, identical sampling devices 1, 1′ and 1″ all containing previouslydescribed tracer sections and adsorbent sections, as is shown in FIG. 7.The sampling devices 1, 1′ and 1″ are arranged in the same housing 10and are oriented in 60° angle with respect to each other. Hence, theareas contributing to the inflow into the permeable units isproportional to sin(α), where α is the angle of the inlet with the flowdirection. If only the concentration of the solute in the surroundingmedium is of concern, only the one sampling device through which mostwater has passed during installation, has to be analysed for solute andtracer mass displaced into this unit. The tracer sections may containcolour or fluorescent dye so that easily can be distinguished for whichof the sampling devices α is most close to 90°. If both the soluteconcentration and the flow direction of the fluid are of concern, allthree permeable units should be analysed for the tracer amounts, and theflow angle may be derived from the relative amounts of tracer displaced.

It would also be possible to arrange two or more sampling devices 1 inthe same housing 10, said sampling devices containing different tracersand or adsorbent matrix materials, in order to measure different typesof chemicals. In this case the sampling devices are not angularlydisplaced in the housing with respect to each other.

In high flow liquid environments with shifting flow direction, such assteams, rivers, drinking water storage containers, or tidal areas, theorientation of the inlet and outlet of the device towards the flowdirection may be achieved by giving the device a special hydrodynamicshape comprising a rudder member 17 as is illustrated in FIG. 9. Thehousing itself may also be given a hydrodynamic shape. This shapeguarantees that the hydrodynamic resistance of the device is at minimumwhen the inlet of the device is upstream. Hence, the inlet of the devicewill always be upstream, even if the flow direction is changing duringthe installation time. The inlet openings 3 of the sampling devices 1may be dimensioned such that the liquid velocity of the liquid in thesampler is reduced, similar to FIG. 6.

It is recognised that the hydraulic conductivity of the permeable unitsalso may be used to regulate the flow rate through the permeable unit.

As the volume fraction of the liquid in partially saturated media is ofparamount importance for the transport of volatile compounds, asimultaneous measurement of the solute flux, water flux and volumetricfluid content would be of large practical value for the quantificationof the gas and solute flux in the porous medium. The measurement of thevolume fraction of the liquid in the porous medium can be achieved byassembling the device as a central metal core or tube, with an isolatedmetal wire wrapped around the central metal core, connecting the centralcore and the isolated wire to a coaxial cable, and measuring the travelspeed of a high frequency electromagnetic block or pulse wave throughthe quasi-coaxial system that the measuring unit represents. Thevelocity of the electromagnetic wave is related to the dielectricconstant of the surrounding porous medium, that in turn is related tothe volume fraction of the liquid in the surrounding medium. Thistechnique, known as TDR (Time Domain Reflectometry) has become a verypopular monitoring instrument for measurements of water contents in thevariously saturated zone. Different geometrical configurations of TDRprobes do exist, all of which in principle may be used in combinationwith the invention. Such a combined probe would then be used tosimultaneously measure the accumulated solute flux and the dynamics ofthe water content in the soil.

Like mentioned before, the complete adsorption of the solutes ofinterest in the permeable device is a prerequisite for proper soluteflux measurements, however this is in practice not a trivial task. Therequirements for the adsorbent materials are:

-   -   very high sorption affinity for the solute of interest    -   Rapid sorption kinetics, so that sorption equilibrium is        attained during the passage of the fluid through the device.    -   Excellent extraction efficiency, which means that the solutes of        interest can be completely extracted from the adsorbent material        in the laboratory with standard laboratory techniques.    -   Good wetting properties for the liquid passing the permeable        device. For example, if the adsorbent would be water-repellent,        the hydraulic conductivity of the device would be strongly        reduced, rendering these adsorbents not useful for applications        in aqueous systems.

Hexagonal Mesoporous Silica (HMS, sometimes called MCM for MobilComposite Materials) is a uniquely structured silicon-oxygen polymer(that may or may not contain aluminium or other metal atoms) that issynthetisized using the templated sol-gel method (U.S. Pat. No.5,215,737). Its special structural properties include very high specificsurface area (up to 800 m²g⁻¹) and hexagonal arrays of ‘wormhole’ shapednanopores. These pores are a direct result of the synthesis method,which is based on the self-assembly of silica monomers around rod-likemicelles formed through hydrophobic interactions between the amine (thetemplate) chains. Although the synthesis of HMS materials is simple, thelattice growth depends on many different parameters, such as type oftemplate, solvent composition, temperature, stirring regime, ageingtime, and silicon or aluminium precursor(s) employed.

Specifically to the current invention, to give one example, thecalcination step was found to have a dramatic effect on the sorptioncapacity of the material towards phenanthrene. The preparation of HMSmaterials with specific structural variables lies somewhere betweenscience and art. That being said, once a particular formulation forpreparation of an HMS material has been established, reproducing similarbatches is not difficult. The high activity of HMS materials materialsin the sorption of non-polar compounds, including phenanthrene,N-propyl-N-[2-(2,4,6-trichlorophenoxy)ethyl]imidazole-1-carboxamide(Prochloraz, a fungicide), di(2-ethylhexyl)phthalate (DEHP, plasticsoftener) and polar compounds, e.g. 4-(2-dodecyl)-benzensulfonic acid,Na salt (LAS, surfactant), makes them ideal candidates for the presentinvention. What is more, these materials are biologically andenvironmentally inert, and in fact are composed almost entirely ofsilicon and oxygen.

Although HMS is an ideal candidate for the use as adsorbent matrixmaterial other adsorbents may be more suited for specific solutes.Further examples of adsorbents that may be used are silica, aluminiumsilicate, aluminium zirconium, metal oxides, synthetic ion exchangeresins, carbonaceous materials, zeolites, carbohydrates and syntheticpolymers, for example polystyrene, polyethylene, polytetra fluoroethylene.

Preferred examples of metal oxides are transition metal oxides, forexample zirconia, titania, and/or lanthanide metal oxides (for exampleceria) with the general formula AxByCzOw, where O is oxygen and A, B andC are transition metals or lanthanides.

Preferred examples of carbohydrates are polysaccharides, for examplecellulose.

Two or more sampling devices may be installed at different locations,especially different depths, in the medium to be investigated. This isillustrated in FIG. 10. These sampling devices are after the samplingperiod removed from the fluid medium and analysed separately. It willthen be possible to detect in which location(s) of the medium a fluidflow and/or certain solutes 18 are present. Different tracer materialsand/or adsorbent matrix material may be used in the different samplingcasings to measure different types of solutes.

The invention is not limited the shown embodiments but can be varied ina number of ways without departing from the scope of the appended claimsand the arrangement and the method can be implemented in various waysdepending on application, functional units, needs and requirements etc.

1. A device for measuring fluid flow and solute mass transport in flowsystems, comprising a casing (1) having inlet (3) and outlet (9)openings and a fluid passageway therebetween, said casing containing atleast one fluid permeable insoluble adsorbent matrix (7) and at leastone tracer material (5, 5′), characterized in that the tracer material(5,5′) is a fluid permeable partially soluble material which at leastprior to installation is not physically or chemically bonded to theadsorbent matrix (7).
 2. A device as claimed in claim 1, characterizedin that the tracer material (5,5′) is located in at least one section(4,4′) of the casing (1) separate from but in contact with at least onesection (6,6′) of the casing holding said insoluble adsorbent matrix(7,7′) and that the at least one tracer material section (4,4′) and theat least one adsorbent matrix section (6,6′) are located in the fluidpassageway between said inlet (3) and outlet openings (9).
 3. A deviceas claimed in claim 2, characterized in that it comprises at least twotracer material sections (4,4′) spaced apart and an adsorbent matrixsection (6) therebetween, which is in contact with said at least twotracer material sections (4,4′).
 4. A device as claimed in claim 3,characterized in that the tracer materials (5,5) in the at least twotracer material sections (4,4′) are chemically different.
 5. A device asclaimed in claim 2, characterized in that it comprises at least twoadsorbent matrix sections (6,6′) spaced apart and with a tracer materialsection (4) therebetween, which is in contact with said at least twoadsorbent matrix sections.
 6. A device as claimed in claim 1,characterized in that the tracer material (5) and the adsorbent matrix(7) are located in the same section in the casing (1), wherein thetracer material and the adsorbent matrix are mixed with each other.
 7. Adevice as claimed in claim 1, characterized in that the tracer material(5, 5′) is chosen from the following groups of materials: inorganic,organic and hybrid organic/inorganic salts; organic, inorganic or hybridorganic/inorganic solids, including polymers, copolymers, blockcopolymers and oligomers in which hydrolysis of certain bonds can leadto the loss of part of the material; microencapsulated materials inwhich the tracer with controlled rate is released from the encapsulationinto fluids to be measured.
 8. A device as claimed in claim 7,characterized in that the tracer material (5, 5′) is a salt having asolubility product (K_(sp)) in the fluid in question of between 10⁻² and10⁻⁶⁰, preferably between 10⁻² and 10⁻⁴⁰ and more preferably between10⁻⁵ and 10⁻¹².
 9. A device as claimed in claim 8, characterized in thatthe tracer material (5, 5′) is chosen from the following group of salts:CaF₂, Ca-Citrate, CaHPO₄, Ca-oleate and Ca-laurate.
 10. A device asclaimed in claim 6, characterized in that at least one recovery standardmaterial is adsorbed to the at least one insoluble adsorbent matrix (7).11. A device as claimed in claim 1, characterized in that the adsorbentmatrix material (7,7′) is an organic, inorganic or hybridorganic/inorganic material having adsorbent properties.
 12. A device asclaimed in claim 11, characterized in that the adsorbent matrix material(7,7′) is chosen from the following groups of materials: silica,aluminium silicate, aluminium zirconium, metal oxides, synthetic ionexchange resins, carbonaceous materials, zeolites, carbohydrates,synthetic polymeric materials.
 13. A device as claimed in claim 12,characterized in that the adsorbent matrix material is a hexagonalmesoporous silica.
 14. A device as claimed in claim 1, characterized inthat the casing (1) comprises an inlet section (2) and an outlet section(8) adjacent said inlet and outlet openings (3) and (9) respectively,said inlet and outlet sections being located outside and presenting aninterfacial area (B) to said adsorbent matrix and/or tracer materialsections (6) and (4), and wherein the projected area (A) normal to theflow direction that contributes to momentum flow into the permeable unitthrough said inlet opening (3) differs in size from said interfacialarea (B) between the inlet section (2) and the adsorbent section (6) orthe tracer section (4).
 15. A device as claimed in claim 1,characterized in that the casing (1) is arranged in a housing (10) whichis impermeable or permeable to fluid, said casing being removable fromthe housing.
 16. A device as claimed in claim 15, characterized in thattwo or more devices each comprising a casing (1, 1′ and 1″) with atleast one adsorbent matrix section (6) and at least one tracer materialsections (4) or a combined adsorbent matrix and tracer material sectionrespectively, and inlet (3) and outlet (9) openings are arranged in thesame housing (10).
 17. A device as claimed in claim 16, characterized inthat said casings (1, 1′ and 1″) are oriented angularly displaced withrespect to each other.
 18. A device as claimed in claim 16,characterized in that said casings (1, 1′ and 1″) contain differenttracer materials (5) and/or adsorbent matrix materials (7).
 19. A deviceas claimed in claim 15, characterized in that the housing (10) isrotatably connected to a core portion (14) having magnetic properties,said core portion (14) being rotatably connected (15) to a cable or rod(16) intended for the installation of the device.
 20. A device asclaimed in claim 15, characterized in that the housing (10) is providedwith at least one member (17) having a hydrodynamic shape and/or thehousing itself having a hydrodynamic shape.
 21. A device as claimed inclaim 15, characterized in that the housing (10) is part of ahigh-frequency waveguide configuration that connects to a coaxial cable.22. A method of measuring fluid flow and solute mass transport in flowsystems, comprising: installing in a medium having a fluid path thereina device comprising a casing (1) having inlet (3) and outlet (9)openings and a fluid passageway therebetween, said casing containing atleast one fluid permeable insoluble adsorbent matrix (7,7′) and at leastone tracer material (5,5′), allowing fluid to pass from said inletopening through the adsorbent matrix and tracer material to said outletopening, characterized in that the tracer material (5,5′) is a fluidpermeable partially soluble material which at least prior toinstallation is not physically or chemically bonded to the adsorbentmatrix (7) and that upon fluid contact the tracer material will dissolvein the fluid and that after a sampling period the device is removed fromthe flow system and the amount of tracer material residing in theadsorbent matrix is quantitatively measured to derive therefrom thefluid flow through the device.
 23. A method as claimed in claim 22,characterized in that the tracer material (5,5′) is located in at leastone section (4,4′) of the casing (1) separate from but in contact withat least one section (6,6′) of the casing holding said insolubleadsorbent matrix (7,7′), so that upon fluid contact the tracer materialwill dissolve in the fluid and be displaced into the adsorbent matrixsection, and that after a sampling period the device is removed from theflow system and the amount of tracer material displaced into theadsorbent matrix is quantitatively measured to derive therefrom thefluid flow through the device.
 24. A method as claimed in claim 23,characterized in that the amount of tracer material (5, 5′) displacedinto the adsorbent matrix (7, 7′) in a direction opposite to the flowdirection is quantitatively measured to compensate for diffusioncontribution to the tracer mass displacement.
 25. A method as claimed inclaim 23, characterized in quantitatively measuring amount of tracermaterial (5, 5′) remaining in the tracer section (4,4′) and comparingwith the amount of tracer material displaced into the adsorbent matrix(6,6′) to derive therefrom diffusion contribution.
 26. A method asclaimed in claim 22, characterized in that the tracer material (5,5′)and the adsorbent matrix (7) are located in the same section in thecasing (1), wherein the tracer material and the adsorbent matrix aremixed with each other, and that upon fluid contact the tracer materialwill dissolve in the fluid and that after a sampling period the deviceis removed from the flow system and the amount of tracer material leftin the casing is quantitatively measured to derive therefrom the fluidflow through the device.
 27. A method as claimed in claim 26,characterized in that that the tracer material (5,5′) is a dispersedinorganic, organic or hybrid inorganic/organic salt of which thepositive cation and negative anion do not have sorption affinity forsaid at least one insoluble adsorbent matrix (7), such that upon fluidcontact the tracer salt will dissolve into the fluid according to itssolubility product, and that after a sampling period the device isremoved from the flow system and the amount of tracer material left inthe casing is quantitatively measured to derive therefrom the fluid flowthrough the device.
 28. A method as claimed in claim 22, characterizedin quantitatively measuring solute adsorbed to the adsorbent matrix toderive therefrom the concentration of solutes in the fluid flow.
 29. Amethod as claimed in claim 28, characterized in quantitatively measuringrecovery standard material adsorbed to the adsorbent matrixsimultaneously with quantitatively measuring solutes adsorbed to thematrix, to derive therefrom a more precise measurement of the amount ofsolute adsorbed to the adsorbent.
 30. A method as claimed in claim 22,characterized in installing in said medium at different locationsthereof, especially different depths, two or more devices eachcomprising a casing (1) having inlet (3) and outlet (9) openings and atleast one adsorbent matrix (7,7′) and at least one tracer material(5,5′), and that after a sampling period the devices are removed fromthe medium and analysed separately.
 31. A method as claimed in claim 30,characterized in that the different devices installed contain differenttracer materials (5,5′) and/or adsorbent matrix materials (7,7′).